#include<stdio.h>
#include<unistd.h>
#include<fcntl.h>

int 
main(int argc, char *argv[]){
    
    
    int fd;

    if((fd = open("t_write_understand.txt",O_RDWR|O_CREAT|O_APPEND,0666))<0){
        
        perror("open error");
    }
    
    lseek(fd,0,SEEK_SET);
    write(fd,"option",3);
    return 0;
}
